Scour at Bridge Foundations on Rock (2012)

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1 Scour at Bridge Foundations on Rock The Interstate Highway (I-90) Bridge over Schoharie Creek in New York failed dur- ing a flood in 1987, leading FHWA to issue a mandate for all highway bridges over water to be evaluated for scour-critical conditions. As state departments of transportation complied with the mandate, a number of bridges were found to have shallow founda- tions bearing on rock. Available procedures for evaluating scour of sand-bed channels produced scour-depth estimates that seemed to be unrealistic or unbelievable for rock materials. This resulted in recognition of the need for improved methods for evaluating scour at bridge foundations on rock, as well as the placement of a number of bridges on the “scour critical” list. An immediate potential benefit of the procedures developed for this research may be removal from the scour-critical list of at least some bridges with shallow foundations on rock. Scour at bridge foundations traditionally is evaluated by hydraulic engineers with input from engineering geologists and geotechnical engineers. NCHRP Project 24-29 focused on recognition of rock and rock-like materials that may be susceptible to scour processes and characterization of bridge foundation conditions in terms that accurately reflect the scour susceptibility and can be used by hydraulic engineers to calculate design scour depths. In essence, the research strives for geotechnical site characterization expressed in scour-relevant terms for use by hydraulic engineers. The guidelines and methods that resulted from this research provide tools for bridge owners and their technical staff members to use in evaluating rock-scour modes to deter- mine which of them might be relevant to particular bridges. The following four modes of rock scour are defined in this research: 1. Dissolution of soluble rocks, 2. Cavitation, 3. Plucking of durable jointed rock blocks, and 4. Gradual wear of degradable rock material. The time between flood events can contribute to reduction in scour resistance through weathering or slaking of rock materials, or enhanced circulation of water in joints held open by gravel fragments wedged into the joint planes during turbulent flow causing blocks of durable rock to vibrate or jostle. Procedures are provided for estimating time-rate of scour and design scour depths for progressive and cumulative scour of degradable rock materials. Guidance also is provided for threshold-controlled scour processes of cavitation and quar- rying and plucking of durable rock blocks. The recommended procedures are relatively simple and quantitative and use equipment and methods familiar to transportation agency personnel and other bridge owners. S u m m a r y

2 Scour at Bridge Foundations on rock One of the most important conclusions of this research is that the scour resistance of degradable rock materials is not solely a function of the rock properties—it is a rock-water interaction phenomenon. The hardest of rock materials will wear away in response to sus- tained powerful discharges, whereas the softest of rock materials may resist erosion for a long period of time in response to tranquil water flow. Waterjet cutters are used to strip concrete away from reinforcing steel for bridge-deck rehabilitation, and the waterjets will cut the steel if applied to the steel for sufficient periods of time. Soluble rock material that dissolves in engineering time is not used for foundation support of bridges. Therefore, the scour-related issue regarding dissolution of soluble rocks is the complex-scour-response of the heterogeneous earth materials that may fill solution cavities. Typical cavity-filling earth material is blocks of relatively hard rock (limestone, dolostone, or marble) in a soil matrix that commonly is clay. The clay will wear away progressively, whereas the rock blocks will be plucked as the threshold conditions are attained. In some cases, loose blocks of rock may accumulate in the filled cavity until they form a self-armoring layer. Cavitation has produced spectacular scour holes in rock in spillway tunnels. However, natural open channels where bridges are likely to be located typically do not have the water depth and velocity conditions needed for cavitation to occur. Thus, a simple check of expected maximum flow depth and velocity may be used to determine if cavitation is likely or even possible. In natural channels where bridges typically are located, hydraulic condi- tions for cavitation do not appear likely to occur. Plucking of durable rock blocks is governed by the size and shape of the rock blocks, the hydraulic loading at peak discharge, and turbulence intensity fluctuations created by flow around bridge piers and across irregularities at block edges on the channel. The Compre- hensive Scour Model applied to open channels provides some guidance on the velocity at the onset of block plucking; this model appears to be promising, but model calibration, which could not be done as part of this research, is needed. Calibration should be done with natural, blocky, rock-bed channel sites; alternatively, flume tests could be conducted to validate and refine the model for applicability to bridge sites on natural channels. The Headcut Erodibility Index and the Erodibility Index Methods were evaluated as part of this research. These two threshold-controlled index methods are similar and both were developed for unlined spillway channels with significantly more hydraulic energy than exist on normal natural channels where bridges are likely to be located. The results of these two index methods applied to bridges evaluated in this research show that the hydraulic energy generally is insufficient for erosion of the local rock masses. Progressive scour of degradable rock material was documented at three of the bridge sites discussed in this report, and a fourth bridge was documented to have experienced no mea- surable scour over a period of several decades. The progressive nature of scour in susceptible rock materials indicates that cumulative hydraulic loading needs to be considered; stream power is a hydraulic parameter that captures flow velocity, flow depth, and slope, and logi- cally can be accumulated over time. Stream power is calculated from commonly available daily flow series, so the accumulated hydraulic parameter is cumulative daily stream power. This cumulative parameter could be converted to unit energy (kW-hr/m2), but the calcula- tions (shear stress × velocity) can be expressed conveniently in terms of unit energy dissipa- tion (ft-lb/s/ft2) which appears to have meaningful units. A probability-weighted approach was used to convert conventional flood-frequency events into event-based scour depths by using stream power and channel response based on observed scour from repeated cross-sections or approximated from specialized geo-

Summary 3 technical laboratory test results. Durations of flows associated with flood frequencies must be included in the analyses. The annualized scour depths associated with the spectrum of flood-frequency events can be combined to produce the time-rate of scour, which is one of the objectives of this research. The service life of the bridge in years times the average annual scour depth produces the design scour depth, which is another research objective. The results of this research can be applied with the greatest confidence to the scour mode of progressive wear of degradable rock material at sites of existing bridges with repeated cross-sections. For such bridges, past scour depths are documented and can be compared to cumulative daily stream power to produce an empirical scour number (i.e., scour depth per unit of stream power). Without repeated cross-sections to document past scour, the analysis must rely on the results of the modified slake durability test. These test results appear to provide a promising opportunity to quantify rock-bed channel response based on an index property of the rock material expressed in stream-power units. Only one of the bridge sites studied gives an opportunity for calibration of the geotechnical test results with repeated cross-section data (SR-273 at the Sacramento River in Redding, California). A second bridge site (Interstate 10 at the Chipola River in Jackson County, Florida) pro- vides a limiting condition of no measurable scour with calculated cumulative daily stream power that has been reconciled with the geotechnical test data to explain why no measurable scour has occurred. Additional bridge sites are needed for calibration and validation of this promising procedure. Repeated cross-sections are the best way to document scour. The cross-sections used in this research posed some challenges for interpretation because the surveys were not well documented and inconsistent locations along the bridge were used for channel depth mea- surements. The value of repeated cross-sections would be improved if a larger number of measurements were taken at consistent locations. It would be helpful to be able to differenti- ate scour at piers from scour between piers.